1. Technical Field
The present invention generally relates to electrically isolated control modules. More particularly, the invention relates to an electrically isolated control module having dual insulation layers between an internal line voltage and an external heat sink.
2. Discussion
Electronic devices operating on relatively high AC line voltages have existed for years. Power tools such as miter saws, belt sanders, and household appliances are but a few examples of these types of devices. The typical power tool has a voltage system providing the line voltage for the tool (based on a commercial AC source) and a tool housing enclosing the system. Control modules (such as a speed controller) are often connected to the line voltage and are also disposed within the tool housing. Thus, in the case of a miter saw, the speed controller may control the rotational speed of the saw blade based on the line voltage and one or more switching signals. In order to avoid electric shock to individuals touching the outside of the tool housing, such a controller is typically embedded into an internal enclosure and positioned far enough away from the tool housing to provide the desired amount of electrical isolation between the tool housing and the line voltage.
While the above-described approach to electrically isolating a control module is acceptable under some circumstances, certain difficulties remain. For example, as the size of power tools and the amount of available airflow within the tool decreases, the flexibility in positioning the module within the tool also decreases. This fact is particularly troublesome considering the exacting requirements of modern day safety standards. For example, Underwriters Laboratories (UL) requires a minimum of 3700 volts of isolation between the AC line voltage and any user-accessible (or user-graspable) housing of a tool or appliance. Design engineers are encountering considerable difficulty in meeting this requirement given the creepage and clearance constraints dictated by these safety standards. It is therefore desirable to provide an electrically isolated module that is highly flexible with respect to module placement in relation to user-accessible housings.